You probably want to check the ChromeOS subreddit for VictoryGoth's bi-weekly buying advice.
Chromebooks $300 or Less
- Acer Chromebook C740
- Acer Chromebook C771 (Intel Celeron. COMING SOON w/ Touch & Non-Touch models)
- Acer Chromebook 14
- Acer Chromebook 15 CB5-571-C4G4
2-in-1 Convertibles $350 or Less
- Acer Chromebook R11
- Acer Chromebook R13
- Asus Chromebook Flip C100 OR the Asus Flip C101 (COMING SOON) if you can wait a few months
- Asus Chromebook Flip C213
You should consider m3/i3 processors or pentiums at worst and consider to spend more money, avoid eMMC and go with NVME/SATA if possible.
There is GalliumOS which you can install on many Chromebooks and runs a bit faster on such low powered hardware, a used Thinkpad or similar offereings from HP or Dell might get you more bang for the bucks.